Optical grating filters in guided-wave optics have been extensively investigated, because they are essential for applications in wavelength division multiplexing systems. When surface-relief gratings are inscribed on waveguides, the grating-waveguides can act as filters to select particular signals from many arriving signals. The desired characteristics of the filter can be achieved by the selection of parameters of the waveguide and the grating. For optical filter applications, high-resolution and high aspect ratio grating fabrication is important because they impact the filtering characteristics and compact size of the devices. Polymeric optical devices are widely used in guided-wave optics owing to their low cost and simple fabrication process. Realization of Bragg gratings in polymer waveguides has attracted much attention in optical communications and optical sensing systems. Typical techniques for patterning gratings on polymer films include holographic lithography (refer to the papers: D. Y. Kim, S. K. Tripathy, L. Li, and J. Kumar, “Laser-induced holographic surface relief gratings on nonlinear optical polymer films,” Appl. Phys. Lett. 66, 1166-1168 (1995); J. W. Kang, M. J. Kim, J. P. Kim, S. J. Yoo, J. S. Lee, D. Y. Kim, and J. J. Kim, “Polymeric wavelength filters fabricated using holographic surface relief gratings on azobenzene-containing polymer films,” Appl. Phys. Lett. 82, 3823-3825 (2003); and S. Aramaki, G. Assanto, G. I. Stegeman, and M. Marciniak, “Realization of integrated Bragg reflectors in DANs-polymer waveguides,” J. Lightwave Technol. 11, 189-1195 (1993).), electron-beam (e-beam) lithography (refer to the paper: H. Nishihara, Y. Handa, T. Suhara, and J. Koyama, “Electron-beam directly written micro gratings for integrated optical circuits,” in Photo- and Electro-Optics in Range Instrumentation, J. Water, et al., eds., Proc. SPIE, 134, 152-159 (1980).), laser beam direct writing (refer to the paper: L. Eldada, C. Xu, K. M. T. Stengel, L. W. Shacklette, and J. T. Yardley, “Laser-fabricated low loss single-mode raised-rib waveguiding devices in polymers,” J. Lightwave Technol. 14, 1704-1713 (1996).), and phase mask lithography (refer to the papers: L. Eldada, S. Yin, C. Poga, C. Glass, R. Blomquist, and R. A. Norwood, “Integrated multichannel OADMS using polymer Bragg grating MZIS,” IEEE, Photonics Technol. Lett. 10, 1416-1418 (1998); and L. Eldada, R. Blomquist, M. Maxfield, D. Pant, G. Boudoughian, C. Poga, and R. A. Norwood, “Thermooptic planar polymer Bragg grating OADM's with broad tuning range,” IEEE Photonics Technol. Lett. 11, 448-450 (1999).). However, few researchers have focused on fabricating surface-relief grating on tunnel waveguides. It has recently been shown that surface-relief gratings can be simply transferred to polymer waveguides by O2 reactive ion etching using azobenzene polymers as the etching mask. But, for these techniques surface scattering loss is often induced due to the surface roughness caused by the physical etching process, and high aspect ratio of the grating patterns is not easy to be obtained by this process (refer to the papers: B. Darracq, F. Chaput, K. Lahlit, Y. Levy, and J.-P. Boilot, “Photoinscription of surface relief grating on azo-hybrid gels,” Advanced Materials 10, 1133-1136 (1998); and D. J. Kang, J. K. Kim, and B. S. Bae, “Simple fabrication of diffraction gratings by two beam interference method in highly photosensitivity hybrid sol-gel films,” Opt. Express 12, 3947-3953 (2004))). The electron-beam direct-writing method has been used to inscribe the polymeric ridge waveguide with a corrugated sidewall Bragg grating (refer to the paper: L. Zhu, Y. Huang, W. M. J. Green, and A. Yariv, “Polymetric multi-channel bandpass filters in phase-shifted Bragg waveguide gratings by direct electron beam writing,” Opt. Express 12, 6372-6376 (2004)). This design is superior to the conventional buried grating for controlling the effective index modulation. It also showed good transmission dip for very short grating length. However, the core size must be very small to have the single mode condition, since the core index of their waveguide is much larger than the surrounding cladding index. This condition will cause the coupling difficulty between the waveguides and ordinary fibers. Ahn et al fabricated Bragg grating filters using the nanoimprint technique (refer to the papers: D.-H. Kim, W.-J. Chin, S.-S. Lee, S.-W. Ahn, and K.-D. Lee, “Tunable polymeric Bragg grating filter using nanoimprint technique,” Appl. Phys. Lett. 88, 071120, (2006)). In their approach, they fabricated a UV transparent quartz stamp and using a nanoimprint machine to successfully transfer the grating pattern onto the polymer layer. The process is cost effective and results in simplicity to fabricate a stamp. But, there are some drawbacks that have been explicitly mentioned in Ref. 19. These drawbacks may restrict the use of this method in fabricating a Bragg grating filter. Kocabas et al reported the fabrication of a grating on OG 146 polymer using e-beam direct writing and stamp transfer techniques (refer to the paper: A. Kocabas and A. Aydinli, “Polymeric waveguide Bragg grating filter using soft lithography,” Opt. Express 14, 10228-10232 (2006)). Then, a BCB polymeric ridge waveguide was fabricated on the grating using reaction ion etching technique. The grating fabrication process is similar to our previous work except for the e-beam writing technique (refer to the paper: W. C. Chuang, C. T. Ho, and W. C. Wang, “Fabrication of a high resolution periodical structure using a replication process” Opt. Express 13, 6685-6692 (2005)). The experimental results showed good replication for the grating through the process. However, the physical etching process may cause large scattering losses from the sidewall of waveguides.
We have recently demonstrated a process to rapidly produce submicron range gratings by using both micro-molding and holographic interference techniques. A large aspect ratio of 0.7:1 between the depth and the period on the grating pattern could be obtained, and consistent reproduction of the grating on a UV polymer could be achieved with this process (refer to the paper: W. C. Chuang, C. T. Ho, and W. C. Wang, “Fabrication of a high resolution periodical structure using a replication process” Opt. Express 13, 6685-6692 (2005)). In this paper, we demonstrate a method to inscribe surface-relief gratings on polymer tunnel waveguides without any physical etching process.
There are a number of simple methods to fabricate polymer waveguides that include techniques involving photocrosslinking (refer to the paper: Jae Wook Kang, Jang-Joo Kim, Jinkyu Kim, Xiangdan Li, Myong-Hoon Lee, “Low-loss and thermally stable TE-mode selective polymer waveguide using photosensitive fluorinated polyimide”, IEEE Photonics Technol. Lett. 14, 1297-1299 (2002)), photobleaching (refer to the papers: T. E. Van Eck, A. J. Ticknor, R. S. Lytel, and G. F. Lipscomb, “Complementary optical tap fabricated in an electro-optic polymer waveguide”, Appl. Phys. Lett. 58, 1588-1590, (1991); and O. Watanabe, M. Tsuchimori, “Improvement in linear and nonlinear optical-properties by blending poly(N-vinyl-2-pyrrolidone) with an electro-optic polymer”, Polymer 42, 6447-6451 (2001)), reactive ion etching (refer to the papers: M. Hikita, Y. Shuto, M. Amano, R. Yoshimura, S. Tomaru, and H. Kozawaguchi, “Optical intensity modulation in a vertically stacked coupler incorporating electro-optic polymer”, Appl. Phys. Lett. 63, 1161-1163 (1993); and W. Wang, D. Chen, and H. R. Fetterman, “Travelling wave electro-optic phase modulator using cross-linked nonlinear optical polymer”, Appl. Phys. Lett. 65, 929-931 (1994)), photolocking (refer to the paper: B. L. Booth, “Low loss channel waveguides in polymers”, J. Lightware Technol. 7, 1445-1453 (1989)) and laser/electron beam writing (refer to the papers: L. Eldada and L. W. Shacklette, “Advances in polymer integrated optics”, IEEE J. Select. Topics Quantum Electron 6, 54-68 (2000); and Y. Y. Maruo, S. Sasaki, and T. Tamamura, “Embedded channel polyimide waveguide fabrication by direct electron beam writing method”, J. Lightwave Technol 13, 1718-1723 (1995)). Some techniques have inherent limitations; for example reactive ion etching can incur excessive scattering losses (refer to the papers: M. Hikita, Y. Shuto, M. Amano, R. Yoshimura, S. Tomaru, and H. Kozawaguchi, “Optical intensity modulation in a vertically stacked coupler incorporating electro-optic polymer”, Appl. Phys. Lett. 63, 1161-1163 (1993); and W. Wang, D. Chen, and H. R. Fetterman, “Travelling wave electro-optic phase modulator using cross-linked nonlinear optical polymer”, Appl. Phys. Lett. 65, 929-931 (1994)), and laser beam writing is not suitable for mass-production (refer to the paper: L. Eldada and L. W. Shacklette, “Advances in polymer integrated optics”, IEEE J. Select. Topics Quantum Electron 6, 54-68 (2000)). Other techniques such as hot embossing (refer to the paper: Holger Becker and Wolfram Dietz, “Microfluidic devices for TAS applications fabricated by polymer hot embossing,” in Microfluid Devices and Systems, A. B. Frazier and C. H. Ahn, eds., Proc. SPIE 3515, 177-181 (1998)), UV-embossing (refer to the paper: P. M. Ferm and L. W. Shacklette, “High volume manufacturing of polymer waveguides via UV-Embossing,” in Linear, Nonlinear, and Power-Limiting Organics, E. Manfred, et al., eds., Proc. SPIE 4106, 1-10 (2000)), and micro-transfer molding method are also becoming more popular due to their simple fabrication procedure (refer to the papers: K. E. Paul, T. L. Breen, J. Aizenberg, and G. M. Whitesides, “Maskless Photolithography: embossed photo-resister as its own optical element,” Appl. Phys. Lett. 73, 2893-2895 (1998); and X.-M. Zhao, S. P-Smith, S. J. Waldman, G. M. Whitesides, and M. Prentiss, “Demonstration of waveguide couplers fabricated using microtransfer molding,” Appl. Phys. Lett. 71, 1017-1019 (1997)). However, these methods have problems to overcome; such as residual material problems and limited substrate and core materials available.